Russian police reported on Sunday that a Siberian doctor who treated Alexei Navalny, President Vladimir Putin’s most prominent critic, after he collapsed on a flight in Russia last year has gone missing.

Physician Alexander Murakhovsky had left a hunting base in a forest on an all-terrain vehicle on Friday and had not been seen since, said the police in the Omsk region, about 2,200 km (1,370 miles) east of Moscow.

It added that drones, a helicopter, emergency services and volunteers on the ground had joined the search effort.

Murakhovsky was the head doctor at the hospital in the Siberian city of Omsk that treated Kremlin critic Navalny. The physician was subsequently promoted to the post of regional health minister.

After tense negotiations with the authorities, Navalny was airlifted to Germany from Omsk for further treatment.

Laboratory tests in 3 European countries, confirmed by the global chemical weapons watchdog, established that Navalny had been poisoned with a Soviet-style Novichok nerve agent.

The Kremlin has repeatedly rejected any suggestion that Russian authorities tried to kill Navalny. He was incarcerated in February on what he said were trumped up charges.